1. Field of the Invention
The present invention relates to a polarizing plate, a polarizing plate having a reflecting layer and a polarizing layer, and a liquid crystal display device using such a polarizing plate.
2. Description of the Related Art
Liquid crystal display devices have found applications in various fields including not only notebook type personal computers, word processors and monitors of desktop type personal computers, calculator, watch, but also liquid crystal projectors, liquid crystal TV sets, digital cameras, electronic notebooks, personal digital assistances, amusement equipment, stationery equipment, cellular phones, car navigation systems, and household electrical appliances such as rice cookers, air conditioners and microwave ovens.
With this widespread use of liquid crystal display devices, a variety of requirements have been made for polarizing plates as a requisite component for the liquid crystal display devices.
As a polarizing plate, generally known is one constructed of an iodine-based polarizing film or a dye-based polarizing film made by adsorbing iodine or a dichroic direct dye to a polyvinyl alcohol resin film in an oriented manner, with transparent protection films such as triacetyl cellulose films laminated to both sides of the polarizing film. The thickness is about 6 μm to about 30 μm for the polarizing film made of a polyvinyl alcohol resin, and is totally about 100 μm to about 190 μm including the transparent protection films. This polarizing film is produced by uniaxially stretching a polyvinyl alcohol resin film, adsorbing iodine or a dichromatic direct dye having an azo group to the film in an oriented manner, and soaking the resultant film in an aqueous solution containing boric acid.
Demand for lighter and thinner devices has been made for portable liquid crystal display devices such as cellular phones, electronic dictionary and personal digital assistances. With this demand, thinner polarizing plates have been required.
Reflective polarizing plates and semi-transmission reflective polarizing plates are often used for portable liquid crystal display devices. As reflecting plates or semi-transmission reflecting plates used for reflective polarizing plates and semi-transmission reflective polarizing plates, often used are scattering-type reflecting plates or semi-transmission reflecting plates constructed of a matted film having a thickness of about 50 μm with aluminum or silver deposited to one side of the film, and mirror-reflection reflecting plates or semi-transmission reflecting plates combined with a light diffusion layer. However, reflective polarizing plates including such reflecting plates are thick and for this reason easy to cause parallax. Thinner reflective polarizing plates are therefore requested.
As a thin polarizing plate, a polarizing plate having a polarizing layer formed by coating a solution containing a dye to a substrate has been known. For example, Japanese Laid-Open Patent Publication No. 3-54506 (JP 3-54506 A) discloses a polarizing plate having a polarizing layer formed by rubbing and then corona-treating a substrate and coating the treated surface of the substrate with a dichromatic dye having a rod molecular shape to arrange the dye molecules on the substrate in the coating direction. This polarizing plate, however, failed to be commercialized because the polarizing performance was insufficient. U.S. Pat. No. 6,049,428 discloses a polarizing plate improved in polarizing performance from the conventional coating-type polarizing plate. This polarizing plate is formed by coating a solution containing a dye to a substrate, which solution is prepared by introducing at least one hydrophilic groups to the dye having a tabular molecular shape and solving the resultant dye to water. However, the polarizing performance of this polarizing plate is still insufficient. Further improvement in transmittance and contrast is therefore requested.